1. Field of the Invention
The present invention pertains to an abrasive article for polishing glass and a method of using the same.
2. Discussion of the Related Art
Glass articles are extensively found in homes, offices and factories in the form of lenses, prisms, mirrors, CRT screens, and other items. Many of these glass surfaces are used in optical components which require that the surface is optically clear and has no visible defects and/or imperfections. If present, defects, imperfections and even minute scratches can inhibit the optical clarity of the glass article. In some instances, these defects, imperfections and/or minute scratches can inhibit the ability to accurately see through the glass. Thus it is desired that the glass surface be essentially free of any defect, imperfection and/or scratch.
Many glass components are curved or contain a radius associated therewith. These radii and curves are generally generated in the glass forming process. However, as a result of the glass forming process, defects such as mold lines, rough surfaces, small points and other small imperfections, can be present on the outer surface of the glass. These defects and/or imperfections, however small, tend to affect the optical clarity of the glass. Abrasive means have been widely used to remove such imperfections and/or defects. The abrasive means typically fall within three main categories: grinding, fining and polishing.
The grinding step further perfects the desired curve or radius and removes any casting defects by rough grinding the glass component with an abrasive tool. Typically this abrasive tool contains a superhard abrasive particle such as a diamond, tungsten carbide or cubic boron nitride. The resulting glass surface is usually of the approximate curvature required. However, the abrasive tool in this rough grinding process will impart coarse scratches into the glass surface such that resulting glass surface is neither precise enough nor smooth enough to directly polish to an optically clear state.
The second step is called "fining." The purpose of the fining step is to refine the coarse scratches generated by the rough grinding process. In general, the fining process will remove the deep scratches from rough grinding and provide a substantially smooth, although not polished surface. The fining process should also result in sufficient removal of the coarse scratches such that the glass surface can be polished to an optically clear surface. If the fining process does not remove all the coarse scratches, then it can be extremely difficult for the polishing step to remove these scratches to obtain an optically clear surface.
This fining process is typically done with a loose abrasive slurry. The loose abrasive slurry comprises a plurality of abrasive particles dispersed in a liquid medium such as water. The most common abrasive particles used for loose slurries are pumice, silicon carbide, aluminum oxide and the like. The loose abrasive slurry may optionally contain other additives such as dispersants, lubricants, defoamers and the like. In most instances, the loose abrasive slurry is pumped between the glass workpiece and a lap pad, such that the loose abrasive slurry is present between the glass workpiece and the lap pad. The lap pad can be made from any material such as rubber, foam, polymeric material, metal, steel and the like. Typically, both the glass workpiece and the lap pad will rotate relative to each other. This fining process typically comprises one or more steps, with each step generating a progressively finer and finer surface finish on the glass workpiece. A finer surface finish can be achieved by various means including utilizing a "softer" abrasive particle, a smaller abrasive particle, "softer" lap pad material and/or different machine conditions. The surface finish of the optical component after this fining process is typically anywhere from about 0.06 to 0.13 micrometer (Ra) and/or an Rtm greater than about 0.30 to 0.90 micrometer.
The roughness of a surface is typically due to scratches or a scratch pattern, which may or may not be visible to the naked eye. A scratch pattern can be defined as a series of peaks and valleys along the surface. Rtm and Ra are common measures of roughness used in the abrasives industry, however, the exact measuring procedure can vary with the type of equipment utilized in surface roughness evaluation. As used herein, Rtm and Ra measurements are based on procedures followed with the Rank Taylor Hobson (Leicester, England) profilometer, available under the trade designation SURTRONIC 3.
Ra is defined as an average roughness height value of an arithmetic average of the departures of the surface roughness profile from a mean line on the surface. Measurements are taken at points both above and below the mean line on the surface within an assessment length set by the Rank Taylor Hobson instrument. Ra and Rtm (defined below) are measured with a profilometer probe, which is a 5 micrometer radius diamond tipped stylus and the results are recorded in micrometers (.mu.m). These departure measurements are totaled and then divided by the number of measurements to arrive at an average value. Generally, the lower the Ra value, the smoother the finish.
Rt is defined as the maximum peak-to-valley height. Rtm is the average, measured over five consecutive assessment lengths, of the maximum peak-to-valley height in each assessment length. In general, the lower the Rtm value, the smoother the finish. A slight variation in the Ra and Rtm values can, but not necessarily, occur when the measurement on the same finished glass surface is performed on different brands of commercially available profilometers.
The third step of the overall process is the polishing step which generates the smoother, optically clear surface on the glass article. In most instances, this polishing step is done with a loose abrasive slurry, since the loose slurry typically provides the means to generate an optically clear surface that is essentially free of any defects, imperfections and/or minute scratches. Typically, the loose abrasive slurry comprises ceria abrasive particles dispersed in water.
Although loose abrasive slurries are widely utilized in the fining and polishing steps of providing an optically clear surface finish on glass articles, loose abrasive slurries have many disadvantages associated with them. These disadvantages include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and dispose of or recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, the steps which use the slurry are usually very untidy because the loose abrasive slurry, which is a viscous liquid, splatters easily and is difficult to contain.
Understandably, attempts have been made to replace the loose abrasive slurry fining and polishing steps with lapping coated abrasives to some degree of success. In general, a lapping coated abrasive comprises a backing having an abrasive coating bonded to it. This abrasive coating comprises a plurality of abrasive particles dispersed in a binder. For example, U.S. Pat. No. 4,255,164 (Butzke et al.), U.S. Pat. No. 4,576,612 (Shukla et al.), U.S. Pat. No. 4,733,502 (Braun) and European Patent Application No. 650,803 disclose various abrasive articles and polishing processes. Other references that teach lapping coated abrasive articles include U.S. Pat. No. 4,644,703 (Kaczrnarek et al.), U.S. Pat. No. 4,773,920 (Chasman et al.) and U.S. Pat. No. 5,014,468 (Ravipati et al.). However, lapping coated abrasives have not completely replaced loose abrasive slurries. In some instances the lapping coated abrasives do not provide a surface which is optically clear and essentially free of defects, imperfections and/or minute scratches. In other instances, the lapping coated abrasives require a longer time to polish the glass article, thereby making it more cost effective to use a loose abrasive slurry. Similarly in some instances, the life of a lapping coated abrasive is not sufficiently long to justify the higher cost associated with the lapping coated abrasive in comparison to loose abrasive slurries. Thus in some instances, lapping coated abrasives are not as economically desirable as loose abrasive slurries.
What is desired by the glass industry is an abrasive article that does not exhibit the disadvantages associated with a loose abrasive slurry, but that is able to effectively and economically polish a glass surface in a reasonable time to optical clarity such that the glass surface is essentially free of imperfections, defects and/or scratches.